Periodic gratings on solar cell back reflectors are an alternative to randomly textured surfaces to provide absorption enhancement. Theoretically, it is impossible to excite quasi-guided modes at every wavelength for a given grating geometry and the broad band enhancement can only be achieved by strong absorption peaks at several wavelengths. Therefore, the critical issue is how to maximize the short circuit current using a limited number of discrete quasi-guided modes. In this work, a common dielectric-semiconductor-dielectric-metal solar cell structure is investigated. It is found that although the number of guided mode peaks has pronounced effect on the solar cell short circuit current, the geometry resulting in the highest short circuit current does not coincide with the geometry leading to the most supported modes. It is also found that high-Q modes are always resulted from global optimization for TE incidence, while low-Q modes are resulted for TM incidence on one-dimensional gratings without a dielectric spacer. Besides, a properly designed and configured dielectric spacer can provide >40% improvement in short circuit current. It is therefore suggested for solar cells with metallic back reflectors, dielectric spacer should be included, and the texture should be formed on the dielectric spacer itself rather than on the metal. Finally, the optimization of the mode quality is proved to be critical in all cases, in addition to the number of supported modes. V

The anti-reflection coating(ARC) based on dielectric nano-particles has been recently proposed as a new way to achieve the low reflectance required for solar cell front surfaces. In this scenario, the Mie modes associated with the dielectric nano-particles are utilized to facilitate photon forward scattering. In this work, versatile designs together with systematically optimized geometry are examined, for the ARCs based on dielectric scatterers. It is found that the Si3N4-TiO2 or SiO2-TiO2 stack is capable of providing low reflectance while maintaining a flat and passivated ARC-semiconductor interface which can be beneficial for reduced interface recombination and prevent V(OC) degradation associated with topography on the active materials. It is also confirmed that the plasmonic nano-particles placed at the front side of solar cells is not a preferred scheme, even with thorough geometrical optimization. At the ultimate design based on mixed graded index(GI) Mie-scattering, the averaged reflectance can be as low as 0.25%. Such a low reflectance is currently only achievable by ultra-long silicon nano-tips, but silicon nano-tips introduce severe surface recombination. On the other hand, the mixed GI Mie design preserves a flat and passivated ARC-silicon interface, with total thickness reduced to 279.8 nm, much thinner than 1.6 μm for silicon nanotips. In addition, the light trapping capability of mixed GI Mie design is much better than silicon nanotips. In fact, when compared to the state-of-art TiO2 light trapping anti-reflection coating, the mixed GI Mie design provides same light trapping capability while providing much lower reflectance.

In this work, a shape-optimized periodic pattern design is employed to boost the short circuit current of solar cells. A decent result of an additional 16.1% enhancement in short circuit current is achieved by solely patternwise optimization, compared to the baseline structure that is already under full parameter optimization. The underlying physics is that the shape-optimized pattern leads to optimal quasi-guided mode excitations. As a result of the pattern design, a single strongly confined quasi-guided mode is replaced with several weakly confined modes, to cover a broader spectral range. Previous works of optimized periodic gratings result in gradually varied grating heights and require grayscale lithography leading to high process complexity. Using randomized pattern for isotropic Lambertian light trapping, on the other hand, leads to an overly large simulation domain. The proposed pattern design methodology achieves the optimal balance between the slow-light enhancement strength and the enhancement spectral range for nanophotonic light trapping using quasi-guided modes.

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